In the injection molding art, it is desirable to monitor the process repeatability of the injection molding machine in a "real time" manner to discover the existence of perturbations that may adversely affect product quality. Current technology requires the addition of transducers of one type or another, plus the appropriate signal conditioning equipment to measure one or more process parameters for this purpose. While good results have been obtained using this approach, several factors including the cost of the required hardware, the fragility of the transducers, the calibration required when product changes occur and the maintenance costs resulting from abuse during normal production operations have discouraged the widespread use of such monitoring systems by injection molding operators.
We have found an alternative process stability monitoring method and apparatus requiring no transducers nor any significant additional hardware. It is based upon our discovery that the operation of the plasticating screw found in virtually all injection molding machines varies in a measurable way with all common process perturbations encountered in the injection molding process. In most injection molding machines, the time required for the screw to plasticate and position material for the next shot "floats" while other machine parameters occur for fixed times. As a result, the screw recovery time is a bulk indicator, the duration of which is affected by the sum total of many operating parameters such as injection pressure, holding pressure, back pressure, melt temperature, injection time, holding time, cycle time, the material being plasticated and the mechanical condition of key components in the molding machine. Our experiments have shown that perturbations in these parameters produce measurable affects in screw recovery time. Thus, by tracking this metric statistically and analyzing it, using any of a plethora of existing statistical methods and displays, processing anomalies can be detected at levels below which the product may be affected. And, because of the time frame involved, identification of the cause may frequently be determined by a simple inspection of a display of measured screw recovery time values.
The method and apparatus of this invention is relatively inexpensive and simple to implement because screw recovery time is easily measured by electrically monitoring a pair of contact points which are usually easily accessible on an injection molding machine. Moreover, these same contact points could be used to obtain the same data which is usually fed to a management computer which presently monitors a different contact closure. Thus, by shifting the management computer connection to the contacts monitored for the screw recovery time metric, it becomes possible to receive additional information over the same pair of wires.
It is, therefore, an object of this invention to provide a method and an apparatus for monitoring the stability of the operation of an injection molding machine.
It is a further object of this invention to provide a method and an apparatus of the type recited which will be simple in design, low in cost and easy to implement on existing injection molding equipment.